Chapter 12

ASK Demodulator

12-1 : Curriculum Objectives

1. To understand the operation theory of ASK demodulation.

2. To understand the operation theory of ASK asynchronous detector.

3. To understand the operation theory of ASK synchronous detector.

4. To understand the methods of testing and adjusting the ASK demodulation circuit.

12-2 : Curriculum Theory

In chapter 11, we have ment ioned that we need a modulator to modulate the data to a

high carrier frequency, so that the signal can be transmitted effectively. Therefore, for receiver,

we must convert the digital signal back to the modulating signal. Figure 12-1 shows the

theoretical diagram of ASK demodulation. There are two methods to design the ASK

demodulator, which are asynchronous detector and synchronous detector. We will discuss

these two types of ASK demodulator in this chapter.

1. Asynchronous ASK Detector.

Figure 12-2 is the block diagram of asynchronous ASK detector. This structure is a typical

asynchronous ASK detector. When the ASK signal pass through the rectifier, we can obtain the

positive half wave signal. After that the signal will pass through a low-pass filter and obtain an

envelop detection. Then get rid of the DC signal, the digital signal will be recurred.

Figure 12-3 is the circuit diagram of asynchronous ASK detector, which R1, R2 and

µA741 comprise an invert ing amplifier to amplify the input signal. Then D1 is the

rectifying diode to make the modulation signal passes through D 1 half wave rectifier. R3

and C1 comprise a low-pass filter. µA741, VR1 , D2, R4 and C2 comprise a comparator,

therefore, the output terminal can demodulate the digital demodulated signal.

Figure 12-2 Block diagram of asynchronous ASK detector.

Figure 12-3 Circuit diagram of ASK asynchronous detector.

2. Synchronous ASK Detector

We have mentioned before that we can use synchronous detector to design the ASK

demodulation. This experiment utilizes the structure of square-law detector and the block

diagram is shown in figure 12-4. Let XASK(t) be the ASK modulated signal, which is

In equation (12-1), the values of amplitude A i have M types of possible change, the ωc

and ϕ0 denote the cutoff frequency and phase constant, respectively. When we input the ASK

modulated signal to the two input terminals of balance modulator, then the output signal of

balanced modulator can be expressed as

Figure 12-5 Internal circuit diagram of MC1496 balanced modulator.

Figure 12-6 Circuit diagram of ASK synchronous detector.

where k represents the gain of balanced modulator. The first term of equation (12-2) is

the data signal amplitude and the second term is the 2nd harmonic of the modulated signal.

From the output signal xout(t)if the first data signal amplitude receives the demodulated ASK

signal, this means that the data signal can be recovered correctly.

In this chapter, we utilize MC1496 balanced modulator to design the square-law

detector as shown in figure 12-5. Figure 12-5 is the internal circuit diagram of MC1496

balanced modulator (Readers may refer to the circuit diagram in chapter 11).

Figure 12-6 is the circuit diagram of synchronous ASK detector. In figure 12-6, Q1, C1,

C2, R2, R3 and R4 comprise an emitter follower. VR1controls the input ranges of modulated

ASK signal and the output signal of MC1496 (pin 12) is shown in equation (12-2). The C9, C11

and R13 comprise a low-pass filter, which the objective is to remove the 2nd

harmonic of

modulated ASK signal as shown in the second term in equation (12-2). The first term in the

equation (12-2) is the data signal amplitude part, which can be recovered by using the

comparator and voltage limiter comprised by µA741, VR2, D1 and D2.

12-3 : Experiment Items

Experiment 1: Asynchronous ASK detector (XR 2206)

1. Use the ASK modulator in chapter 11 with R, = 1 M (as shown in figure 11-3) or refer to

figure DCT 11-1 on GOTT DCT-6000-06 module to produce the amplitude modulated signal as

the modulated ASK signal input. Let J2 be short circuit and J3 be open circuit.

2. At the data signal input terminal (Data I/P) in figure DCT11-1, input 5V amplitude and 100

Hz TTL signal.

3. Connect the ASK signal output terminal (ASK O/P) in figure DCT11 -1 to the signal input

terminal of the asynchronous ASK detector (ASK I/P) in figure DCT 12-1.

4. Adjust the variable resistor VR 1 in figure DCT12-1 to obtain the optimum reference level

of the comparator. By using oscilloscope, observe on the output signal waveforms of the

negative feedback amplifier (TP1), demodulated signal output port (TP2), comparator

reference level (TP3) and the digital signal output port (Data O/P). Finally, record the

measured results in table 12-1.

5. According to the input signal in table 12-1, repeat step 2 to step 4 and record the measured

results in table 12-1.

6. Use the ASK modulator in chapter 11 with R, = 510 Ω (as shown in figure 11 -3) or refer to

figure DCT 11-1 on GOTT DCT-6000-06 module to produce the amplitude modulated

signal as the modulated ASK signal input. Let J2 be open circuit and J3 be short circuit.

7. According to the input signal in table 12-2, repeat step 2 to step 4 and record the

measured results in table 12-2.

Experiment 2: Asynchronous ASK detector (MC 1496)

1. Use the ASK modulator in chapter 11 (as shown in figure 11 -6) or refer to figure DCT11-2

on GOTT DCT-6000-06 module to produce the amplitude modulated signal as the modulated

ASK signal input.

2. At the data signal input terminal (Data I/P) in figure DCT11-2, input 5 V amplitude and 100 Hz

TTL signal. At the carrier signal input terminal (Carrier I/P), input 400 mV amplitude and 20

kHz sine wave frequency.

3. Adjust VR1 of ASK modulator in figure DCT11-2 and observe on the modulated ASK signal

before the signal occurs distortion, then slightly adjust VR 2 to avoid the asymmetry of the

signal to obtain the optimum output waveform of modulated ASK signal (ASK O/P ).

4. Connect the ASK signal output terminal (ASK O/P) in figure DCT11-2 to the signal input terminal

of the asynchronous ASK detector (ASK I/P) in figure DCT 12- 1.

5. Adjust the variable resistor VR 1 in figure DCT12-1 to obtain the optimum reference level

of the comparator. By using oscilloscope, observe on the output signal waveforms of the

negative feedback amplifier (TP1), demodulated signal output port (TP2), comparator

reference level (TP3) and the digital signal output port (Da ta O/P). Finally, record the measured

results in table 12-3.

6. According to the input signal in table 12-3, repeat step 3 to step 5 and record the measured

results in table 12-3.

7. At the data signal input terminal (Data I/P) in figure DCT11-2, input 5V amplitude and 100

Hz TTL signal. At the carrier signal input terminal (Carr ier I/P), input 400 mV amplitude

and 100 kHz sine wave frequency.

8. According to the input signal in table 12-4, repeat step 3 to step 5 and record the measured

results in table 12-4.

Experiment 3: Synchronous ASK detector

1. Use the ASK modulator in chapter 11 (as shown in figure 11 -6) or refer to figure DCT 11-2 on

GOTT DCT-6000-06 module to produce the amplitude modulated signal as the modulated ASK

signal input.

2. At the data signal input terminal (Data I/P) in figure DCT11-2, input 5V amplitude and 1 kHz

TTL signal. At the carrier signal input terminal (Carrier I/P), input 400 mV amplitude and

100 kHz sine wave frequency.

3. Adjust VR1 of ASK modulator in figure DCT11-2 and observe on the modulated ASK

signal before the signal occurs distortion, then slightly adjust VR2 to avoid the asymmetry

of the signal to obtain the optimum output waveform of modulated ASK signal (ASK O/P).

4. Connect the ASK signal output terminal (ASK O/P) in figure DCT11 -2 to the signal input

terminal of the asynchronous ASK detector (ASK I/P) in figure DCT 12-2.

5. By using oscilloscope and switching to DC channel, then adjust VR 2 of figure DCT12-2 to

obtain the optimum comparator reference voltage. Then observe on the output signal

waveforms of the emitter follower (TP1), balanced modulator (TP2), comparator (TP3) and data

signal output port (Data O/P). Finally, record the measured results in table 12-5. If the signal

output waveform occurs distortion, then slightly adjust VR1.

6. According to the input signal in table 12-5, repeat step 3 to step 5 and record the measured

results in table 12-5.

7. At the data signal input terminal (Data I/P) in figure DCT11-2, input 5 V amplitude and 1

kHz TTL signal. At the carrier signal input terminal (Carrier I/P), input 400 mV amplitude and 40

kHz sine wave frequency.

8. According to the input signal in table 12-6, repeat step 3 to step 5 and record the measured

results in table 12-6.

12-4 : Measured Results

Table 12-1 Measured results of ASK demodulator by using asynchronous detector. (2206 IC, J2

SC, J3 OC)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 100 Hz

TP1

TP2

TP3

Data O/P

Table 12-1 Measured results of ASK demodulator by using asynchronous detector. (continue)

(2206 IC, J2 SC, J3 OC)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 500 Hz

TP1

TP2

TP3

Data O/P

Table 12-2 Measured results of ASK demodulator by using asynchronous detector. (2206 IC, J2

OC, J3 SC)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 100 Hz

TP1

TP2

TP3

Data O/P

Table 12-2 Measured results of ASK demodulator by using asynchronous detector. (continue)

(2206 IC, J2 OC, J3 SC)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 500 Hz

TP1

TP2

TP3

Data O/P

Table 12-3 Measured results of ASK demodulator by using asynchronous detector. (MC 1496,

Vc = 400mV, fc = 20 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 100 Hz

TP1

TP2

TP3

Data O/P

Table 12-3 Measured results of ASK demodulator by using asynchronous detector.(continue)

(MC 1496, Vc = 400mV, fc = 20 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 500 Hz

TP1

TP2

TP3

Data O/P

Table 12-4 Measured results of ASK demodulator by using asynchronous detector. (MC 1496,

Vc = 400mV, fc = 100 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 100 Hz

TP1

TP2

TP3

Data O/P

Table 12-4 Measured results of ASK demodulator by using asynchronous detector.(continue)

(MC 1496, Vc = 400mV, fc = 100 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 500 Hz

TP1

TP2

TP3

Data O/P

Table 12-5 Measured results of ASK demodulator by using asynchronous detector. (MC 1496,

Vc = 400mV, fc = 100 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 1kHz

TP1

TP2

TP3

Data O/P

Table 12-5 Measured results of ASK demodulator by using asynchronous detector.(continue)

(MC 1496, Vc = 400mV, fc = 100 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

Vp = 5 V

fData = 5 kHz

TP1

TP2

TP3

Data O/P

Table 12-6 Measured results of ASK demodulator by using asynchronous detector. (MC 1496,

Vc = 400mV, fdata = 1 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

40 kHz

TP1

TP2

TP3

Data O/P

Table 12-6 Measured results of ASK demodulator by using asynchronous detector. (continue)

(MC 1496, Vc = 400mV, fdata = 1 kHz)

Data Signal

Frequencies

Data I/P ASK O/P

70 kHz

TP1

TP2

TP3

Data O/P

12-5 : Problem Discussion

1. In figure 12-3, if we neglect the µA741 op-amp and connect the ASK modulator to the

diode detector, then what are the results?

2. What are the purposes of the comparators in figure 12-3 and figure 12-6?

3. In figure 12-6, what are the objectives of R13, C9 and C11?

4. In figure 12-6, what are the objectives of D1 and Dz?